1. Field of the Invention
This invention relates to a zinc-rich water-based paint that is used for a painting or coating, while mixing zinc powders with an alkali silicate as a binder before use or while mixing a binder in a previously prepared zinc powder containing slurry.
2. Description of the Related Art
Paints or coatings utilizing a sacrificial corrosion protection of a zinc to an iron are widely used as zinc-rich paints. However, most of them contain an organic solvent, so that there is a problem that they have a bad influence on the environment. On the other hand, there is a problem in an organic solvent free zinc-rich water-based paint that it produces hydrogen gas by a mixture of the zinc and a water or that it is inferior in a rust preservation capability. A zinc-rich paint containing a chromic acid as a binder is also widely used. However, there is also a problem that it has a bad influence on the environment since it contains the chromic acid.
In view of such situation, the inventor of the present invention has invented a process of obtaining a zinc-rich water-based paint and filed a patent application therefor in Japan as Japanese Patent Application No. 2002-304863. In the invention, an ion-exchange water is added to a solution of an alkali silicate so as to hydrolyze it and lower a polymerization degree thereof. Then, zinc powders are mixed in the solution. Thus, a dense coating of a silica is formed on a surface of a zinc powder, thereby preventing contact of the zinc and the water. Consequently, the zinc powders are given high stability in water. As a result, a zinc powder containing slurry is obtained at low costs, while being given the high stability. A lithium silicate is mixed as a binder in the slurry and their mixture is stirred, so that a zinc-rich water-based paint is prepared. The Japanese Patent Application has been laid open by Japanese Patent Office on Jan. 8, 2004, as Japanese Laid-Open Patent Publication No. 2004-2637. That is, the publication date of 2004-2637 is later than a priority date of the present patent application, Nov. 25, 2003. Thus, the publication 2004-2637 is not a prior art of the present application. Still, the publication 2004-2367 is cited herein since it discloses contents relating to the present invention.
If the lithium silicate is used as the binder, a stable zinc-rich water-based paint is obtained. However, the lithium silicate is expensive and is inferior in adhesive property. Therefore, a potassium silicate or a sodium silicate has been used instead of the lithium silicate since it is inexpensive and superior in the adhesive property. Still, zinc ions are dissolved in a zinc-rich water-based paint using the potassium silicate or the sodium silicate as the binder after mixing. Such dissolved zinc ions act as a gelling agent of the alkali silicate. Consequently, there is a problem that the stability of the paint and a coating property thereof go down over time.